Converting to 12v

Converting to 12 volts is an attractive proposition. The benefits are brighter lights and the bulbs are more easily available. A 6v Lucas dynamo actually delivers over 20v if you spin it fast enough so you can just fit a 12v regulator and ride off into the sunset with your lights on.... if you ride fast enough!

The popular Lucas E3L dynamo, which was fitted to most British bikes from the early 1950's, delivers about 60 watts, regardless of voltage. At 1400 r.p.m. (for example) it pushes out between 7v and 9v, depending on a number of factors, not least the cleanliness of the brushes and commutator. The type of regulator installed will obviously have an impact on how the charging system works but, generally speaking, we need at least 7v from the dynamo for the regulator to function and, therefore, charge the battery.

On a BSA A10 the dynamo is spinning at engine speed +10%. Depending on the gearing employed a BSA A10 would be doing about 20 m.p.h. in top gear at about 1270 r.p.m. which is approximately 1400 r.p.m. dynamo speed. so, at the lowest possible cruising speed, the battery will charge; this is no coincidence as the BSA designers will have made calculations to achieve a charging system which is effective whilst putting the lowest strain on the dynamo (i.e. not making it spin unnecessarily fast).

Whilst the Lucas dynamo is quite robust it is obviously in its best interests to reduce strain by spinning it as slowly as possible. The bearings, particularly the output side bearing, are small. The armature is not very well balanced and the dynamo is subjected to considerable heat and vibration. Evidence from the workshop suggests that collapse of the small bearing, and commutators throwing segments, are quite common occurrences; in both cases the impact on the dynamo can be catastrophic!

The faster the dynamo spins the less efficient it becomes so, to charge a 12v battery, it may be necessary to cruise at about 45 m.p.h. or more. Imagine a journey at night, perhaps with the occasional period through a town or in speed limited areas..... Or maybe you like to ride with your lights on in daylight which is a requirement in many countries.... And maybe your battery isn't fully charged because you don't use your bike that much..... Or maybe you quite often ride in a group whose speed is restricted by the slowest rider....  It is easy to see how any, or a combination of, these conditions will prevent you from maintaining a reasonable speed over time so your battery will go flat, eventually. Running out of lights at night is no fun and dangerous. Riding faster than you want to, watching your ammeter needle all the time, just to keep the battery charged is also dangerous and riding under this sort of pressure is no fun either!

There are a number of very well-made kits on the market which increase dynamo speed by 10% or even 20%. This will bring your required cruising speed down but will, of course, increase the dynamo speed throughout the range and may be 'the last straw' on the odd occasion when you decide to overtake something, particularly if your dynamo is still running with its original, tired components!

Our 12v conversion is achieved by replacing the internals of the dynamo. We fit a new, UK manufactured armature, field coil and bearings so you get a 14v (approx.) charge at 1400 r.p.m., plenty to charge a 12v battery without increasing the dynamo speed. If you are thinking of having your dynamo refurbished it would be a good idea to consider a 12v conversion at the same time as the difference in cost is minimal.




Comments